Operation through which two spacecraft meet and are physically joined in space.

Doppler effect

Phenomenon that occurs when a source of vibration (sound, for example) or electromagnetic radiation (light, radio waves) at a given frequency is in motion relative to an observer, which results in a shift of the frequency reaching the observer. The Doppler effect is thus used to determine the speed of a moving object emitting sound or light, by measuring the difference between the frequency received and the frequency leaving the source.

Drag compensation

Technique used to compensate for all non-gravitational forces acting on a satellite. Such forces are chiefly due to drag, caused by atmospheric friction against the walls of the satellite.

drop shaft

Vertical underground facility used to perform experiments in weightless conditions.

drop tower

Facility in which experiments are subjected to free-fall for a few seconds to create conditions of weightlessness.

dynamic topography

Variation in the level of the oceans, from a few centimetres to several tens of centimetres, due to ocean circulation. Actual sea level is governed by the combined effects of the dynamic topography and the geoid, which is the static component.